Hydraulic fracturing (also known as well stimulation) is the injection, under pressure, of water, sand, and/or other fluids within a well formation to induce fractures in a rock layer. Oil and Gas drilling operators commonly use hydraulic fracturing, or “fracing” to release petroleum and natural gas as well as other substances from the rock layer. The high pressure injection creates new channels in the rock which can increase the extraction rates and ultimate recovery of fossil fuels. A hydraulic fracturing pump or “frac pump” is used to pump water, sand, gravel, acids, proprietary liquids and concrete into the well formation. The solids pumped down the hole into the fractures keep the fractures from closing after the pressure is released. Operators generally attempt to pump as much volume as possible at or above the pressure necessary to frac the well.
Fracing is very expensive and generally charged by the hour. Because the formation may be located thousands of feet below the earth's surface, the pressures generated and required by frac pumps are substantial, sometimes exceeding 23,000 pounds per square inch (psi). At peak times, a given frac pump may operate for more than eight consecutive hours (with drive engines running) at as much as 2800 revolutions per minute (rpm).
The frac pump is driven by high horsepower diesel or turbine engines. An engine's revolutions-per-minute (RPM's) are usually reduced through the use of a transmission. The transmission is usually multi-geared such that higher pump loads use lower gearing and lighter loads use higher gearing. Internally, a frac pump increases pressure within a fluid cylinder by reciprocating a plunger longitudinally within the fluid head cylinder. A frac pump comprises two major components: a power frame and a fluid head held together by a group of stay rods. Conventional high pressure, high volume frac pumps have either three or five cylinders. A typical power frame consists of a pinion gear, bull gears, crankshaft, rod caps, connecting rods, crossheads and pony rods. Each crosshead and pony rod is maintained in proper position by a respective large brass cylinder pressed into an individual steel support sleeve welded into the power frame. Each crosshead and pony rod is coupled to the respective connecting rod with a separate wrist pin. Each connecting rod is bolted to individual rod caps that are connected to the crankshaft. The crankshaft is connected to either one or two bull gears that are driven in circular motion by a pinion gear.
As mentioned, in most frac pumps, the connecting rod is attached at one end to the crankshaft, and at the other, the crosshead. The crosshead, in turn, is coupled to the pony rod which is connected to a plunger. The connecting rod is connected to the crosshead by a wrist pin inserted through a wrist pin hole positioned in both the connecting rod and the crosshead. Thus, the crankshaft's rotational movement is transferred through the connecting rod into linear movement by virtue of the sliding arrangement of the crosshead within a brass sleeve. This linear movement, in turn, moves the crosshead and pony rod, which in turn moves the plunger in, on pressure stroke and out on suction stroke, in a linear fashion. Because of the extreme conditions under which a frac pump operates, some of which are discussed above, there is considerable wear and tear on the various component parts. Such wear and tear requires constant maintenance, and ultimately, replacement of worn parts. Maintenance and repair creates machine downtime and increases the overall cost of oil and gas production.
Four of the parts that are susceptible to failure and, thus, machine downtime, are the connecting rod, crosshead, pony rod, and the brass sleeve. As a result of its connection with the crankshaft, the connecting rod transfers the radial energy of the crankshaft into linear motion. The connecting rod may become weakened by the off-center stress produced by the crankshaft rotation. A conventional connecting rod comprises a thicker wall that is required due to its overall straight shank design. Such design and configuration of the conventional connecting rod results in increased weight. Weight is a significant factor for frac pump operators as only so much weight is permitted to be carried down the highway. In operation, an increased weight of moving machine components results in increased horsepower demands. This raises the cost of operation.
The conventional connecting rod has a male knuckle end that, on the compression stroke, pushes against a brass bearing located inside a crosshead female knuckle, and on the suction stroke, pulls against the wrist pin. Due to the loss of oil pressure or poor lubrication of this male/female knuckle arrangement, the connecting rod male knuckle is caused to become worn and/or galled by the brass bearing. Such wear to the male knuckle results in failure and need of replacement of the connecting rod. The connecting rod is very expensive and a difficult part to manufacture due to the configuration of the knuckle end and the way it must be machined. The knuckle end must be ground to the proper size and to a very fine finish so as to move properly against the brass bearing. Because the conventional connecting rod knuckle does not comprise a full circle, proper machining of the knuckle is very time consuming and costly as specialized training and equipment is required.
The conventional crosshead is a casting made of cast iron or steel and is a part that is subject to wear. Virtually all castings are subject to fracturing. The crosshead is pushed and pulled within the brass sleeve which maintains the crosshead in linear alignment. The outside diameter of the crosshead and the brass sleeve within which it travels are each subject to significant wear. The crosshead is in direct contact with the brass sleeve. When the crosshead wears and gets a certain amount of play (loss of fit) the crosshead starts knocking. This, in turn, aggravates wear in the brass cylinder. This wear causes a movement in the connected pony rod which also causes movement in the plunger which, in turn, causes premature loss of the pony rod seal and the pressure seal to the plunger. If the fit of the crosshead is the cause of the play, the crosshead must be replaced. If the brass sleeve is found to be worn, it must be replaced.
Crossheads and brass sleeves are very costly and time consuming to replace. To replace the crossheads or brass sleeves, the pump must be removed from the trailer and entirely disassembled.
To replace the brass sleeves, the empty power frame must be placed on a boring mill and the old brass removed. The steel support cylinder must be machined to “true up” or reconfigure the bore to a proper configuration for proper reinsertion of a replacement brass sleeve. A raw brass tube must be cut to length and turned to a proper diameter for the correct size press fit required to hold the sleeve in place. The brass sleeve must then be placed on a milling machine to have holes or slots milled to allow oil drainage from in front of the crosshead. The brass sleeve is then usually placed in liquid nitrogen (although alcohol and dry ice are also used) so that the brass sleeve is reduced to an extremely low temperature and a point at which it shrinks to a size small enough to be placed within the steel support sleeve. This freezing process is dangerous as it involves a fire hazard and the handling of subzero materials. Under favorable conditions, this process usually takes approximately thirty minutes per cylinder. However, upon reinsertion, the replacement brass sleeve sometimes locks up before reaching the proper depth or position and must be cut out and re-replaced at more cost and time. Once the brass sleeve is properly installed, the power frame must again be positioned on a boring mill so that the brass sleeve inside diameter may be bored back to factory specifications. To prevent future oil leakage in this area, the brass sleeve ends must then to be machined so that they are flush with the head plate of the power frame. Replacement of the brass sleeve is the most time consuming portion of the power frame rebuilding process.
As may be seen from the foregoing, in order to replace the brass sleeves, the pump has to be completely disassembled and handled several times on and off different machines. This is a very time consuming, costly and resource intensive operation. To prevent pumping operations from becoming interrupted for prolonged periods of time as a result of brass sleeve replacement, pump owners must purchase more pumps than would otherwise be necessary.
Just as the crosshead and brass sleeves are subject to wear and require replacement, the pony rod is also a part that may require replacement due to wear. The pony rod is clamped to the plunger located in the fluid head. The pony rod is used to transfer the linear motion of the crosshead to the plunger in the fluid head. On the in stroke of the pony rod, the plunger is pulled out causing a suction opening of the intake valve. Such suction opening of the intake valve pulls fracing fluids into the fluid head. On the out stoke of the crosshead, the plunger closes the intake valve and opens an exhaust valve. Such action pushes the fluid in the fluid head out to the wellhead under pressure.
The conventional pony rod is bolted to the crosshead as a separate part. The pony rod transfers energy produced in the crank case to the plunger. Due to its contact with the seal that seals off the pony rod to oil leakage from the power frame and exposure of the pony rod to the outside elements such as dust, acids, fracing fluids, etc., the pony rod is subject to significant wear. The pony rod can also come loose from the crosshead due to the bolts loosening, which can cause catastrophic failure of the frac pump.
In order to extend the life of a pony rod, in the original manufacture, a wear area of the pony rod is undercut, and spray metalized with powdered hard metal. The pony rod is then heat treated so as to bond the metalized surface together. Then the metalized surface is machined in a grinding machine to proper diameter and finish to fit the seals.
After becoming worn, the conventional pony rod can be re-machined. To do so, the old hard surface must be removed by grinding, and re-metalized, heat treated and machined or ground back to proper size. These worn pony rods are usually exchanged for either a rebuilt pony rod or a new one. The metalizing operation of both new and used pony rods requires a large amount of labor, time, and resources. The pump must be disassembled to replace the pony rod. Moreover, a stock pony rod deteriorates each time it is subjected to high temperature by both spraying of metal and heat treatment of the sprayed on surface to make the sprayed metal bond to itself. Each time steel is heated to high temperatures there is a loss of material through oxidation (scale), warping of the metal and softening of the steel. The high heat elongates the grain of the material. Longer grain generally equates to softer steel while smaller grain generally equates to harder steel. The pony rod steel may become soft enough so as to be unable to carry sufficient loads. Also soft steel may result in damage to the clamp end of the pony rod, which calls for complete replacement of the unit. Reworked pony rods have a much higher failure rate. The buyer buys a reworked pony rod to save money but never knows how many times it has been reworked. The buyer may be unaware that a reworked pony rod life is not as long as a new one. In the event of premature failure of a reworked pony rod, the frac pump must again be torn down for replacement. Though conventional pony rods are discarded when a new pump is bought, the pony rods of the present invention may be reused in new pumps. The higher the pressure at which a frac pump runs, the shorter the life of the power frame. Conversely, lower operating pressures extend the life of the power frame. The power frames are heavily stressed during operation. The average power frame is rebuilt up to four times. Power frames running 15,000 lbs. pressure or more last two to three rebuilds. Normal life is between 800 to 1000 hours between rebuilds. The frac pump requires constant monitoring during operation for a multitude of failures that can happen while in operation.
Due to the long rebuild turnaround, operation under less than ideal conditions, and high maintenance costs, frac pump owners inevitably must “over buy” frac pumps to compensate for the number of pumps being out of service. The power frame is the most expensive part of the frac pump. When a power frame is retired, it is sold as junk. An owner replaces the old with a complete new power frame. No old parts are used in the new power frame.
The present invention provides tremendous savings to the user in both rebuild cost and an increase in turnaround time which lowers the quantity of inventory of frac pumps needed to be kept on hand because of rebuilding delays. These delays result from turnaround times that can be as high as six months due to waiting for parts or finding machine time to machine the rebuilds on.
The invention eliminates the need for a large number of brass sleeves to be manufactured and saves on tooling for extra machine work, electricity and many other intangibles.
A replaceable pony rod sleeve is provided that can be replaced in the field without disassembling or “tearing down” the machine. The replacement of the pony rod sleeve is fast and inexpensive compared to the normal replacement method which requires the pump to be taken out of service and completely disassembled. The pony rod sleeve contributes to keeping the pump in the field, thus, lowering operation cost not only because of decreased cost involved with replacement but by reducing the need for more inventory to compensate for rebuilding delays.
The present invention provides large savings in obviating or reducing the need to replace the brass sleeves through which crossheads travel. If either the outside diameter of the crosshead or the inside diameter of the brass sleeve become sufficiently worn such that they would ordinarily require replacement, the present invention, eliminates this requirement by providing replaceable wear bridges that may be used in a re-machined existing brass sleeve. The original brass sleeve is bored out to a minimum clearance and configuration, and replacement wear rings are provided, the wear rings being oversized such that wear rings bring back the original clearance between the two parts. The brass sleeves do not have to be replaced during the normal life of the power frame.
By providing a pony rod as an integral part of the crosshead significant weight is saved and the pony rod cannot come loose as in the conventional crosshead and bolted on pony rod. In the event the pony rod should come loose, a complete loss of the frac pump could occur resulting in the need to replace the entire pump or at least repair major damage.
The main body of the combination crosshead and pony rod may be reused through several generations of new power frames because everything that can wear out can be replaced. There is no further heat treatment of the body combination crosshead and pony rod, other than the original heat treatment, which greatly increases life of the unit. This invention brings a value not previously seen or offered. There is a much lower cost of operation, reusable parts in the next new machine purchase and much faster turnaround times which also has the benefit of the need for fewer pumps in inventory.
The connecting rod provided in this disclosure attaches to the crankshaft in a similar manner as a conventional connecting rod. However, the modified connecting rod of an embodiment of the present invention comprises a tapered shank instead of a straight shank of the conventional design. As the crankshaft rotates the connecting rod is placed under great stress by the off center push of the crankshaft. The tapered shank of the connecting rod is much stronger than the straight design of the stock connecting rod and puts the wall of the connecting rod more in line with the stress of the pressure stroke. The tapered body also allows a thinner wall, because of its design advantage, and is less likely to break or deflect than the straight design with a good strength advantage. The tapered body has a weight advantage. The bearing end of the connecting rod is a thick walled cylindrical cylinder laying perpendicular to the length wise center of the cross head. It is commonly called a knuckle end. This design is used because it offers more square inches of surface area to handle the tremendous pressures involved in the pressure stroke of the frac pump. The knuckle end pushes a brass or plated steel bearing located inside the crosshead. Lubrication travels through an access hole in the top side of the crosshead, then through the bearing. The knuckle end has a small hole drilled through the top center intersecting the larger hole bored thru lengthwise of the knuckle end. This hole allows access of lubrication to lubricate the wrist pin hole. Due to poor lubrication, loss of oil pressure, worn bearing or worn knuckle end of the connecting rod, the metal between the two surfaces, bearing and knuckle end, can gall or deform. A loss of this bearing causes deformation of the knuckle end of the connecting rod. When there is damage to the knuckle end the connecting rod is replaced at a great cost. The stock knuckle end is not a full circle but requires a very smooth and accurate surface. If this surface is not formed to precise specifications, the bearing may be damaged very quickly which leads to premature pump failure.
Grinding the knuckle end cannot be performed in a full circle. The semicircle machining requirement of the conventional connecting rod requires a specialized grinder to grind the radius of the stock knuckle end and maintain the accuracy required of the part. This grinding operation is slow and very costly.
In a preferred embodiment of the present invention, the wear surface of the knuckle end part of the connecting rod is replaceable. The replaceable sleeve is easy to machine and install. The replaceable sleeve does not require any special machinery and is easily made in mass quantities. The sleeve is made as a thin walled cylinder either heat treated, hard chrome plated, or with other plating options. The replaceable sleeve is ground around a full circle which does away with special machinery, speeds up production and greatly lowers the cost of the sleeve. The replaceable sleeve is then cut to fit the modified cut knuckle end. The finish of the replaceable sleeve becomes the part that facilitates smooth operation rather than knuckle end itself.
The design of the present connecting rod is stronger than a conventional connecting rod and less susceptible to breakage. As the knuckle end is replaceable, there will be no loss of the rod because of wear, loss of oil pressure, galling, or deformation. The new connecting rod can be used in the next new frac pump because of its design and replaceable end. There is nothing to wear out the connecting rod. All of these attributes have a very good cost advantage require less labor than conventional connecting rods and provide a savings in terms of natural and manmade materials.